1. Field of the Invention
The present invention relates to a load driving device, and more particularly, to a load driving device including an output transistor that controls power supply to a load.
2. Description of Related Art
Semiconductors for power supply have been widely employed as load driving devices that supply power from a power supply to a load. In one field of application, the semiconductors are used to drive actuators or lamps of vehicles.
In the case of using such a load driving device for vehicles, there is a demand for preventing a wasteful consumption current from occurring when the load driving device is in a standby state, with a standby current on the order of microamperes. In case the power supply is reversely connected by mistake, there is a demand for preventing the load driving device from being destroyed. If the power supply is reversely connected, it is desirable to bring a power device (e.g., an output transistor) into a conduction state, to thereby suppress heat generation in the power device and prevent breakdown of the load driving device.
Japanese Unexamined Patent Application Publication No. 2009-165114 discloses a solution for these demands. FIG. 14 shows a load driving device disclosed in Japanese Unexamined Patent Application Publication No. 2009-165114. An operation of the load driving device when a power supply is normally connected and an operation thereof when the power supply is reversely connected are described with reference to FIG. 14.
When the power supply is normally connected, a positive-polarity-side voltage VB of a power supply 10 is supplied to a power supply terminal PWR. Further, a negative-polarity-side voltage VSS of the power supply 10 is supplied to a ground terminal GND.
When an output transistor T1 is conductive, a transistor MN2 is non-conductive. Specifically, a driver circuit 12 outputs a signal S1 of H level and a signal S2 of L level. At this time, transistors MN6 and MN7 provided in a back gate control circuit 16 are conductive. This is because when the transistor T1 becomes conductive, a potential of an output terminal OUT indicates the H level, and a gate potential of each of the transistors MN6 and MN7 becomes higher than a potential of the ground terminal GND. Further, transistors MN4 and MN5 provided in the back gate control circuit 16 become non-conductive. This is because a gate potential of each of the transistors MN4 and MN5 is equal to the potential of the ground terminal GND.
Thus, the potential of the ground terminal GND is applied to a back gate of a protection transistor MN3 which is provided for reverse connection protection. Then, the protection transistor MN3 becomes non-conductive. As a result, there is no path for discharging the gate charge of the transistor T1, and thus the transistor T1 is made more conductive by the signal S1.
When the output transistor T1 is non-conductive, the transistor MN2 is conductive. Specifically, the driver circuit 12 outputs the signal S1 of L level and the signal S2 of H level. At this time, the transistor MN2 discharges the gate charge of the transistor T1, thereby bringing the transistor T1 into a non-conduction state.
In this case, the transistors MN6 and MN7 provided in the back gate control circuit 16 are conductive while the potential of the output terminal OUT is high. However, when the potential of the output terminal OUT shifts toward the potential of the ground terminal GND, the transistors MN6 and MN7 become non-conductive. Also, the transistors MN4 and MN5 provided in the back gate control circuit 16 become non-conductive. In short, each of the transistors MN4 to MN7 becomes non-conductive. However, since the potential of each of the output terminal OUT and the ground terminal GND is equal to the potential of the ground terminal GND, the back gate of the protection transistor MN3 indicates the potential of the ground terminal GND. Accordingly, the protection transistor MN3 becomes non-conductive.
When the power supply is reversely connected, the positive-polarity-side voltage VB of the power supply 10 is supplied to the ground terminal GND. Further, the negative-polarity-side voltage VSS is supplied to the power supply terminal PWR. When the power supply is reversely connected, the driver circuit 12 and the transistor MN2 cannot operate normally. This is because a parasitic diode between a back gate and a drain of each transistor is forward biased due to the reverse connection of the power supply 10, which makes it impossible for each transistor to operate normally.
The potential of the output terminal OUT first indicates a forward voltage of the parasitic diode of the output transistor T1. When the protection transistor MN3 provided in a reverse connection protection circuit 15 starts supplying electric charges to a gate of the output transistor T1, the transistor T1 becomes conductive. As a result, the potential of the output terminal OUT approaches the potential of the power supply terminal PWR (i.e., a value of voltage drop caused by an on-resistance of the output transistor T1 and a load current). Further, an anode potential of a diode D10 indicates a forward voltage of the diode D10.
A back gate of the transistor MN7, which is provided in the back gate control circuit 16, is not coupled to the GND terminal. Accordingly, the transistor MN7 operates as a reverse-biased diode (backflow prevention diode).
The potential of the output terminal OUT and an anode potential of the diode D10 first indicate a forward voltage (e.g., about 0.6 V) of a diode. Thus, a low potential is applied to the back gate of the protection transistor MN3, and the protection transistor MN3 becomes conductive. As a result, electric charges are supplied to the gate of the transistor T1 from the GND terminal through the protection transistor MN3. Then, a gate voltage of the transistor T1 increases, and the transistor T1 becomes conductive. When the transistor T1 becomes conductive, the potential of the output terminal OUT decreases to about a potential of the power supply terminal PWR (a potential according to the negative-polarity-side voltage VSS) from the forward voltage of the diode. Also in this case, the back gate of the protection transistor MN3 is maintained at a low potential, and thus the protection transistor MN3 maintains the conduction state. Accordingly, the transistor T1 maintains the conduction state. In this manner, the load driving device according to the prior art can suppress heat generation in the transistor T1, thereby preventing breakdown of the load driving device.